1. Field of the Invention
The present invention relates to a sample separating device, more specifically, a device for separating the components of a blood sample from each other.
2. Description of the Related Art
Generally, in the analysis of a blood sample, the blood sample is centrifuged, and the centrifuged components (blood plasma and blood cell, or serum and blood clot) are separately collected. A separating device of this type has an interface detector, and the interface between components adjacent to each other is detected by the interface detector.
FIG. 7 shows the main portion of the separating device. The separating device includes a plasma suction nozzle 4 and a blood cell suction nozzle 5, which are integrally set to a holder 6. Both nozzles 4 and 5 are connected to an impedance detector 7. The nozzles 4 and 5 are made of a conductive material, and serve also as electrodes. The nozzles are also connected to a suction driving source (not shown). The holder 6 is moved downward by a holder driving member (not shown).
As the holder 6 descends, both nozzles 4 and 5 approach a test tube 1, and reach a blood sample 8 in the test tube 1. While the nozzles 4 and 5 are descending, the impedance between the nozzles 4 and 5 is detected by an impedance detector 7. When the output value from the impedance detector 7 reaches a predetermined threshold value for the plasma detection, the holder 6 stops and a plasma component 2 is taken in from the plasma suction nozzle 4.
After the suction of the plasma component, the descent of the nozzles 4 and 5 is started once again. When the output value from the impedance detector 7 reaches a predetermined threshold value for the blood cell detection, the holder 6 stops and a cell component 3 is taken in from the cell suction nozzle 5.
For a simple explanation, the holder driving means for the holder 6 and the comparator are not shown in FIG. 7.
FIG. 8 is a diagram showing a typical example of the relationship between a nozzle position and an output from the impedance detector 7. As the tip end of each of the nozzles 4 and 5 proceeds through the air, the plasma component, and the blood cell, the impedance significantly varies, as shown in the figure. The impedance is at the highest level when the tip ends of the nozzles 4 and 5 are both in the air, and is at the lowest level when in the plasma component 2. When the tips of the nozzles 4 and 5 are located in the blood cell component 3, the impedance is somewhere between the two levels. In the figure, X and Y represent standard plasma impedance and blood cell impedance, respectively.
Before the separation of the components, a threshold value, S.sub.1, for plasma detection and a threshold value, S.sub.2, for blood cell detection are determined. The value S.sub.1 is set lower than the value S.sub.2. The impedance having a value of S.sub.1 indicates that the plasma suction nozzle 4 has reached the plasma component 2. When this indication is detected, the plasma component 2 is taken in. The nozzles 4 and 5 further descend and when the impedance exceeds the value S.sub.2, which indicates that the blood cell suction nozzle 5 has reached the blood cell component 3, the cell component 3 is taken in.
It should be noted here that the variation of the impedance is not always constant for any type of blood samples, but each sample has its own variation characteristic of impedance. If the threshold values S.sub.1 and S.sub.2 are set at a constant for any blood samples regardless of the unique variation characteristic of each sample, it is difficult to accurately detect the interface between a plasma component 2 and a blood cell component 3.
For example, when a detected blood cell impedance Y.sub.1 is much lower than a standard value (indicated by dot line 9a), and is even lower than the threshold value S.sub.2 for a blood cell components, the blood cell component 3 cannot be detected from the S.sub.2 value.
As shown in FIG. 9, a blood cell component 3 having such a low impedance can be detected by setting the S.sub.2 value lower than Y.sub.1. However, in order to detect the plasma 2, the S.sub.1 value must be set even lower than S.sub.2. If the S.sub.1 value is simply set at a low level, S.sub.1 may be lower than X.sub.1. Therefore, in the case where the detected plasma impedance X.sub.1 is much higher than the standard plasma impedance X (dot line 9b), the plasma component cannot be detected.
In general, plasma suction nozzles 4 and 5, which are mass-produced, do not have exactly the same impedance, but the impedance varies from one to another. The impedance of each nozzle also varies with time (i.e. aging of nozzles). Other than the unique characteristic of each blood sample, the above factors cause an increase in the plasma impedance X.sub.1.
The present invention has been proposed in consideration of the above drawbacks of the conventional technique, and the purpose of the invention is to provide a sample separator capable of accurately detecting the interface between components adjacent to each other regardless of the difference between samples in characteristics.